session_ID bug in XnatUtils

```
Traceback (most recent call last):
  File "/Users/rogersbp/miniconda3/envs/dax/bin/Xnatreport_assessors", line 22, in <module>
    Assrs = xnat.list_project_assessors(args.project)
  File "/Users/rogersbp/miniconda3/envs/dax/lib/python3.10/site-packages/dax/XnatUtils.py", line 1176, in list_project_assessors
    sess_id2mod[asse['session_ID']][0]
KeyError: ''
```

